Sunday, November 28, 2010

Upon infection by HIV, there follows a massive but transient increase in viruses in the blood followed by a period of years during which CD4+ T lymphocytes slowly decline. However, after the initial spike, some people have far fewer viruses in their blood and their CD4 counts do not decline. They remain apparently healthy, asymptomatic, for a very long time. These people are called “controllers” because their immune system controls virus replication without medication. The authors of this paper – with an impressive number of collaborators – looked for genetic variations in the controllers that could underlie the differences.

A genome wide association study (GWAS) performed with 1, 974 controllers (cases) and 2,648 progressors revealed a strong link with the HLA (the real human MHC, or major histocompatibility complex) on the short arm of chromosome 6 (figure shown). A weaker correlation with a chemokine receptor CCR5delta32 polymorphism previously identified with HIV resistance was also identified. HLA association is not surprising because practically everything immunological is strongly influenced by the HLA. What the investigators did next, however, was novel, imaginative, and highly illuminating.From the SNP data, they were able to impute the controllers’ HLA type. Of the hundreds of HLA alleles, controllers tended to possess a remarkably narrow number of particular HLA-B and, to a lesser degree, HLA-A and -C alleles. They identified HLA-B*57:01, B*27:05, B*14/Cw*08:02, B*52, A*25 as protective alleles, and B*35 and Cw*07 as risk alleles. Moreover, they identified specific amino acids within the peptide-binding cleft as key variables in HIV control. These HLA amino acids define the ability of a pathogen to stimulate the cellular immune response.

These results provide a solid, well understood albeit complex mechanistic understanding to HIV progression and control. The next step could identify those HIV peptides that are bound preferentially by the protective and risk allele proteins.

The Major Genetic Determinants of HIV-1 Control Affect HLA Class I Peptide Presentation. The International HIV Controllers Study. Science. 2010 Nov 4.

Thursday, October 7, 2010

Glucocoriticoids, including the natural hormone cortisol, are powerful immune suppressants. Synthetic glucocorticoids, such as dexamethasone and prednisone, are often taken by mouth to control autoimmune diseases. However, controlling the autoimmune disease systemic lupus erythematosus (SLE, lupus) often requires more aggressive treatments with high doses of more potent glucocorticoids, such as methylpredisolone, given intravenously.

Lupus is characterized by serum antibodies to nucleic acids (anti-nuclear antibodies, ANA), by a pattern of interferon-alpha-induced genes transcribed ("IFN signature”), and by an increase in plasmacytoid dendritic cells (PDC). PDC help produce antibodies and IFN. The IFN signature and PDC levels in patients are reduced to normal levels by high doses of intravenous glucocorticoids (see Figure 1c).The authors noted that serum nucleic acids, which trigger the production of ANA, also stimulate PDC though specific receptors called toll-like receptors-7 and -9 (TLR-7 & -9) expressed on many cells of the immune system. Further, they hypothesized that TLR stimulation renders the PDC resistant to the suppressive effects of glucocorticoids.

Indeed, purified PDC treated with glucocorticoids (1-10 uM) do not survive overnight in a flask unless they are also treated with a nucleic acid (CpG) that stimulates TLRs (Figure 2a, first panel shown here). Nucleic acid-mediated protection is partially reversed by IRS, a synthetic oligonucleotide inhibitor of TLRs (IRS 954). Nucleic acid-containing immune complexes, isolated from the sera of lupus patients, also protected PDC by triggering TLRs.

Similar results were obtained with PDC from healthy people, suggesting that the PDC of lupus patients are not different but instead PDC are made glucocorticoid resistant by chronic stimulation of TLRs by nucleic acids. The authors conclude that “inhibitors of TLR7 and 9 signaling could prove to be effective corticosteroid-sparing drugs.”

Wednesday, August 4, 2010

There are many strategies to remain healthy and reduce the effects of aging: healthy diet and exercise, yoga, drastically reducing calories, etc... all disappointingly slow and indirect. On the other hand, simply feeding mice the anti-rejection drug rapamycin has been shown to dramatically increase both the average and maximum life span, even when started late in life, probably by postponing death from cancer, by retarding mechanisms of aging, or both. Unfortunately, rapamycin also strongly suppresses the immune system, increasing the risk of a dangerous infection. How can we derive the benefits of rapamycin while avoiding the risks?An endogenous inhibitor of the “Target Of Rapamycin” (TOR), and thus a candidate for the physiological rapamycin-like agent, are sestrins (Sesns), which are conserved proteins that are induced by cellular stress including by elevated adenosine monophosphate kinase (AMPK) or DNA damage.

Drosophila (fruit flies) have a single form of sestrin (dSesn). To define the activities of dSesn, these authors created flies with either gain- or loss-of-function dSesn mutations. Clever sleuthing with agonists and employing other mutant signaling molecules showed that TOR induces dSesn through reactive oxygen species (ROS, quenched by antioxidants such as vitamin E), jun-N-terminal kinase (JNK), and a transcription factor (FoxO). p53 was implicated but narrowly escaped the round-up of usual suspects. dSesn repaid TOR's attention by suppressing its stimulation of tissue growth. Thus dSesn is induced by TOR and suppress TOR activities, constituting a feedback inhibition system. These authors go on to show that flies lacking dSesn suffer multiple age-related pathologies, including muscle degeneration and heart malfunction. These pathologies are prevented by activating AMPK with AICAR (5-aminoimidazole-4-carboxamide 1-b-D-ribofuranoside) or inhibiting TOR with rapamycin (Figure 5C shows that heart arrythmias suffered by dSesn-deficient flies are treated by feeding them AICAR, rapamycin, or vitamin E).

Forget caloric restriction, I'll take my ice cream with a statin mixin and Rapa sprinkles. Or a burger with heaping side order of sestrins.

Monday, July 19, 2010

Elevated white cells in the bloodstream (leukocytosis) was correlated with cardiovascular disease and atherosclerosis nearly a century ago but the cause remains unclear. Elevated high-density lipoprotein (HDL), in contrast, is correlated with protection from cardiovascular disease, in part because HDL carries cholesterol away from macrophages in atherosclerotic plaques. Here, Yvan-Charvet and colleagues report that leukocytosis develops in mice lacking the membrane proteins ABCA1 and ABCG1 that normally transport cholesterol out of macrophages to lipoproteins. These ABCA1/G1-deficient mice suffer a myeloproliferative disorder and display an expansion of particular blood-forming (hematopoietic) stem cells. Although lymphoid (B, T, NK cells) precursor cells were unchanged, myeloid precursor cells, which give rise to granulocytes, macrophages, etc, were doubled. Similar findings in MyD88-knockout mice ruled out innate inflammation as a cause of leukocytosis. Transplantation of bone marrow from ABCA1/G1 transporter-deficient mice into (apoA1-transgenic) mice with elevated HDL blocked or slowed the development of leukocytosis, myeloproliferation, the particular stem cell population, and atherosclerosis. The figure shows that elevated HDL in apoA1-tg mice protected against heart disease (Fig 4B, left side shows the entire hearts and, right side, tissue sections from hosts transplanted with ABCA1/G1-deficient bone marrow; upper panel: diseased heart from a normal recipient mouse showing leukocyte infiltration and, lower panel, healthy heart from a HDL-elevated recipient).

The authors state that “these results suggest that HDL suppresses the proliferation of myeloid progenitor cells by promoting cholesterol efflux”. Thus, these transporters (intrinsically) or elevated HDL (extrinsically) can regulate hematopoiesis and atherosclerosis. They also reported that stimulating ABCA1/G1 expression above normal levels using a transcriptional activator (TO901317) increased cholesterol efflux and suppressed myeloid cell proliferation, suggesting a new therapeutic rationale.

Statins, such as atorvastatin (Lipitor) and rosuvastatin (Crestor), were designed and are prescribed to reduce serum cholesterol, which is associated with cardiovascular disease. Surprisingly, statins were also found to be anti-inflammatory. Could these results help explain statins' anti-inflammatory effects?

Stressing cells infected by Tb has been shown to induce autophagy ("self-eating", free review), increase presentation of Tb antigens on the infected cell surface, and stimulate specific T lymphocytes. This group found that rapamycin, a pharmaceutical drug used to suppress immunity after solid organ transplantation, induces autophagy and improves immune recognition of Tb. This figure shows that rapamycin treatment of dendritic cells causes a >10-fold increase in their ability to induce anti-Tb immunity in mice (Fig. 5e). Most of the cell culture experiments aimed at dissecting this effect show a much more modest effect of extremely high doses of rapamycin (1 mM!). Oddly, many of the early figures measure IL-2, a hallmark of the T lymphocyte response, even though rapamycin's mode of action inhibiting IL-2 activity would seem to confound the interpretation of the results.

Mission

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